"Return web pages updated in the..."

When I am doing a Google Advanced (Web) Search, and use the option to
"Return web pages updated in the...past 3 months, 6 months, year", it seems
to make no difference whatsover to my results when choosing any of the
options. Why is this option provided if it makes no difference? (I do see
that the posting date works with Google group searches, but am only asking
about a Web search).
Thanks